When Red Sox officials flew to Nashville in early December for the Winter Meetings, they were convinced the Yankees would land Johan Santana because of a willingness to include Phil Hughes in an offer.

When the Yankees left the Winter Meetings, having decided to keep Hughes, they were sure the packages brandished by Boston would put Santana alongside Josh Beckett in the Red Sox’s rotation.

Yet yesterday afternoon, the Yankees and Red Sox were bystanders as the Mets completed the biggest score any team made this offseason – and perhaps the biggest trade in their history – by sending four prospects to Minnesota to actually acquire Santana, a two-time Cy Young winner whose presence catapults the Mets not only to NL East favorites, but perhaps to the team to beat in the entire league.

The trade – in which the Mets dealt outfielder Carlos Gomez and pitchers Kevin Mulvey, Philip Humber and Deolis Guerra – is not complete until Santana passes a physical and agrees to an extension. The Mets have until 5 p.m. on Friday to finalize a new contract, and they already were working with Santana’s agent, Peter Greenberg, yesterday. The two sides are slated to have a meeting today with the expectation that the Mets will get a better grasp of Santana’s contractual requests.

The Mets have said they do not want to go beyond five years in an extension. However, the Santana camp has indicated it wants to extend six or seven years at an average annual total of perhaps as high as $25 million. If Santana does not like the offer, he can invoke his no-trade clause, play out the season with the Twins and become a free agent. There have been indications that the Mets might try to induce Santana with easily reachable options.

As difficult as the contract negotiations appear, the Mets already have come further than could have been anticipated when this process began.

The Yankees were perceived as the initial frontrunner, showing a willingness to include Phil Hughes in a package. However, when Andy Pettitte agreed to forgo retirement and return, the Yanks’ sense of urgency for Santana dimmed. That is when the Red Sox seemed to push ahead in this derby, offering packages built around either Jacoby Ellsbury or Jon Lester. The Twins seemed to figure that as long as they had the Yanks and Red Sox in this competition that eventually either the Yanks would break down and include Ian Kennedy along with Hughes or the Red Sox would agree to put Lester and Ellsbury in the same offer.

But once the Yanks stepped back, it became clear Boston was more interested in keeping Santana away from the Yanks than ballooning their payroll to obtain him.

The Twins initially told the Mets there could be no deal without Jose Reyes included. Recently, they insisted both Gomez and Fernando Martinez be packaged together. But the Mets refused both requests.

Late last week, motivated apparently by Santana’s insistence that a deal must be done by spring training, the Twins told the three finalists to make their last, best offers by Monday. When neither the Red Sox nor the Yankees budged, Minnesota began working exclusively with the Mets.

“Everything broke the Mets’ way to get a great player,” an AL official said.

Gomez is the player that most attracted the Twins to the Mets offer. There are questions around baseball if Gomez will ever hit consistently. But Minnesota envisions Gomez as a frontline center fielder.

The Mets, like most of baseball, have no doubts about Santana, who turns 29 in March. With him, the Mets’ rotation moves from questionable to formidable. With Santana anchoring the staff, the other members move down a slot, with Pedro Martinez now the No. 2 starter followed by Oliver Perez, John Maine and Orlando Hernandez. The Mets now have Mike Pelfrey as insurance, especially for the fragile Martinez and Hernandez, rather than battling for a rotation spot.

One AL executive, in lauding the Mets, repeated “he’s the best” about Santana three times before adding, “What he can be in the NL, at a pitcher’s park like Shea, is scary.”

Santana, who went 15-13 with a 3.33 ERA and 235 strikeouts last year, has won between 15-20 games, tossed at least 219 innings and posted a 3.33 ERA or better in each of the last four seasons.

“The Mets got maybe the best pitcher in the game, so I think that’s probably the focus from their end,” an MLB team executive said. “I know they don’t like giving up young prospects but at the end of the day I’m sure they’re not super concerned with what they gave up considering what they got.”

Santana at a glance:

NAME: Johan Alexander Santana

BIRTHDATE: March 13, 1979

POSITION: Left-handed starting pitcher

MAJOR LEAGUE DEBUT: April 3, 2000

RECORD: 93-44

ERA: 3.22

STRIKEOUTS: 1,381

HONORS: American League All-Star, 2005, 2006, 2007; American League Cy Young Award, 2004, 2006